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[PATCH] VFS: update overview document
This patch updates the Documentation/filesystems/vfs.txt document. I rearranged and rewrote parts of the introduction chapter and added better headings for each section. I also added a description for the inode rename() operation which was missing and added links to some useful external VFS documentation. Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Pekka Enberg
Linus Torvalds
parent
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@ -3,7 +3,7 @@
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Original author: Richard Gooch <rgooch@atnf.csiro.au>
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Last updated on August 25, 2005
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Last updated on October 28, 2005
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Copyright (C) 1999 Richard Gooch
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Copyright (C) 2005 Pekka Enberg
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@ -11,62 +11,61 @@
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This file is released under the GPLv2.
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What is it?
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===========
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Introduction
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============
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The Virtual File System (otherwise known as the Virtual Filesystem
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Switch) is the software layer in the kernel that provides the
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filesystem interface to userspace programs. It also provides an
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abstraction within the kernel which allows different filesystem
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implementations to coexist.
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The Virtual File System (also known as the Virtual Filesystem Switch)
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is the software layer in the kernel that provides the filesystem
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interface to userspace programs. It also provides an abstraction
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within the kernel which allows different filesystem implementations to
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coexist.
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VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
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on are called from a process context. Filesystem locking is described
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in the document Documentation/filesystems/Locking.
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A Quick Look At How It Works
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============================
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Directory Entry Cache (dcache)
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------------------------------
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In this section I'll briefly describe how things work, before
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launching into the details. I'll start with describing what happens
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when user programs open and manipulate files, and then look from the
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other view which is how a filesystem is supported and subsequently
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mounted.
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The VFS implements the open(2), stat(2), chmod(2), and similar system
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calls. The pathname argument that is passed to them is used by the VFS
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to search through the directory entry cache (also known as the dentry
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cache or dcache). This provides a very fast look-up mechanism to
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translate a pathname (filename) into a specific dentry. Dentries live
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in RAM and are never saved to disc: they exist only for performance.
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The dentry cache is meant to be a view into your entire filespace. As
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most computers cannot fit all dentries in the RAM at the same time,
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some bits of the cache are missing. In order to resolve your pathname
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into a dentry, the VFS may have to resort to creating dentries along
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the way, and then loading the inode. This is done by looking up the
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inode.
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Opening a File
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--------------
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The Inode Object
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----------------
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The VFS implements the open(2), stat(2), chmod(2) and similar system
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calls. The pathname argument is used by the VFS to search through the
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directory entry cache (dentry cache or "dcache"). This provides a very
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fast look-up mechanism to translate a pathname (filename) into a
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specific dentry.
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An individual dentry usually has a pointer to an inode. Inodes are
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filesystem objects such as regular files, directories, FIFOs and other
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beasts. They live either on the disc (for block device filesystems)
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or in the memory (for pseudo filesystems). Inodes that live on the
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disc are copied into the memory when required and changes to the inode
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are written back to disc. A single inode can be pointed to by multiple
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dentries (hard links, for example, do this).
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An individual dentry usually has a pointer to an inode. Inodes are the
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things that live on disc drives, and can be regular files (you know:
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those things that you write data into), directories, FIFOs and other
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beasts. Dentries live in RAM and are never saved to disc: they exist
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only for performance. Inodes live on disc and are copied into memory
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when required. Later any changes are written back to disc. The inode
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that lives in RAM is a VFS inode, and it is this which the dentry
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points to. A single inode can be pointed to by multiple dentries
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(think about hardlinks).
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To look up an inode requires that the VFS calls the lookup() method of
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the parent directory inode. This method is installed by the specific
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filesystem implementation that the inode lives in. Once the VFS has
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the required dentry (and hence the inode), we can do all those boring
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things like open(2) the file, or stat(2) it to peek at the inode
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data. The stat(2) operation is fairly simple: once the VFS has the
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dentry, it peeks at the inode data and passes some of it back to
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userspace.
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The dcache is meant to be a view into your entire filespace. Unlike
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Linus, most of us losers can't fit enough dentries into RAM to cover
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all of our filespace, so the dcache has bits missing. In order to
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resolve your pathname into a dentry, the VFS may have to resort to
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creating dentries along the way, and then loading the inode. This is
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done by looking up the inode.
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To look up an inode (usually read from disc) requires that the VFS
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calls the lookup() method of the parent directory inode. This method
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is installed by the specific filesystem implementation that the inode
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lives in. There will be more on this later.
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Once the VFS has the required dentry (and hence the inode), we can do
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all those boring things like open(2) the file, or stat(2) it to peek
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at the inode data. The stat(2) operation is fairly simple: once the
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VFS has the dentry, it peeks at the inode data and passes some of it
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back to userspace.
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The File Object
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---------------
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Opening a file requires another operation: allocation of a file
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structure (this is the kernel-side implementation of file
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@ -74,51 +73,39 @@ descriptors). The freshly allocated file structure is initialized with
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a pointer to the dentry and a set of file operation member functions.
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These are taken from the inode data. The open() file method is then
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called so the specific filesystem implementation can do it's work. You
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can see that this is another switch performed by the VFS.
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The file structure is placed into the file descriptor table for the
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process.
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can see that this is another switch performed by the VFS. The file
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structure is placed into the file descriptor table for the process.
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Reading, writing and closing files (and other assorted VFS operations)
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is done by using the userspace file descriptor to grab the appropriate
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file structure, and then calling the required file structure method
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function to do whatever is required.
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For as long as the file is open, it keeps the dentry "open" (in use),
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which in turn means that the VFS inode is still in use.
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All VFS system calls (i.e. open(2), stat(2), read(2), write(2),
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chmod(2) and so on) are called from a process context. You should
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assume that these calls are made without any kernel locks being
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held. This means that the processes may be executing the same piece of
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filesystem or driver code at the same time, on different
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processors. You should ensure that access to shared resources is
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protected by appropriate locks.
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file structure, and then calling the required file structure method to
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do whatever is required. For as long as the file is open, it keeps the
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dentry in use, which in turn means that the VFS inode is still in use.
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Registering and Mounting a Filesystem
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-------------------------------------
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=====================================
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If you want to support a new kind of filesystem in the kernel, all you
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need to do is call register_filesystem(). You pass a structure
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describing the filesystem implementation (struct file_system_type)
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which is then added to an internal table of supported filesystems. You
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can do:
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To register and unregister a filesystem, use the following API
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functions:
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% cat /proc/filesystems
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#include <linux/fs.h>
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to see what filesystems are currently available on your system.
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extern int register_filesystem(struct file_system_type *);
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extern int unregister_filesystem(struct file_system_type *);
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When a request is made to mount a block device onto a directory in
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your filespace the VFS will call the appropriate method for the
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specific filesystem. The dentry for the mount point will then be
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updated to point to the root inode for the new filesystem.
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The passed struct file_system_type describes your filesystem. When a
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request is made to mount a device onto a directory in your filespace,
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the VFS will call the appropriate get_sb() method for the specific
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filesystem. The dentry for the mount point will then be updated to
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point to the root inode for the new filesystem.
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It's now time to look at things in more detail.
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You can see all filesystems that are registered to the kernel in the
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file /proc/filesystems.
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struct file_system_type
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=======================
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-----------------------
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This describes the filesystem. As of kernel 2.6.13, the following
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members are defined:
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@ -197,8 +184,14 @@ A fill_super() method implementation has the following arguments:
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int silent: whether or not to be silent on error
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The Superblock Object
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=====================
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A superblock object represents a mounted filesystem.
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struct super_operations
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=======================
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-----------------------
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This describes how the VFS can manipulate the superblock of your
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filesystem. As of kernel 2.6.13, the following members are defined:
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@ -286,9 +279,9 @@ or bottom half).
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a superblock. The second parameter indicates whether the method
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should wait until the write out has been completed. Optional.
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write_super_lockfs: called when VFS is locking a filesystem and forcing
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it into a consistent state. This function is currently used by the
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Logical Volume Manager (LVM).
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write_super_lockfs: called when VFS is locking a filesystem and
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forcing it into a consistent state. This method is currently
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used by the Logical Volume Manager (LVM).
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unlockfs: called when VFS is unlocking a filesystem and making it writable
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again.
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@ -317,8 +310,14 @@ field. This is a pointer to a "struct inode_operations" which
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describes the methods that can be performed on individual inodes.
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The Inode Object
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================
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An inode object represents an object within the filesystem.
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struct inode_operations
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=======================
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-----------------------
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This describes how the VFS can manipulate an inode in your
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filesystem. As of kernel 2.6.13, the following members are defined:
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@ -394,51 +393,62 @@ otherwise noted.
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will probably need to call d_instantiate() just as you would
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in the create() method
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rename: called by the rename(2) system call to rename the object to
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have the parent and name given by the second inode and dentry.
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readlink: called by the readlink(2) system call. Only required if
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you want to support reading symbolic links
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follow_link: called by the VFS to follow a symbolic link to the
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inode it points to. Only required if you want to support
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symbolic links. This function returns a void pointer cookie
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symbolic links. This method returns a void pointer cookie
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that is passed to put_link().
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put_link: called by the VFS to release resources allocated by
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follow_link(). The cookie returned by follow_link() is passed to
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to this function as the last parameter. It is used by filesystems
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such as NFS where page cache is not stable (i.e. page that was
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installed when the symbolic link walk started might not be in the
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page cache at the end of the walk).
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follow_link(). The cookie returned by follow_link() is passed
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to to this method as the last parameter. It is used by
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filesystems such as NFS where page cache is not stable
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(i.e. page that was installed when the symbolic link walk
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started might not be in the page cache at the end of the
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walk).
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truncate: called by the VFS to change the size of a file. The i_size
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field of the inode is set to the desired size by the VFS before
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this function is called. This function is called by the truncate(2)
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system call and related functionality.
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truncate: called by the VFS to change the size of a file. The
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i_size field of the inode is set to the desired size by the
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VFS before this method is called. This method is called by
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the truncate(2) system call and related functionality.
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permission: called by the VFS to check for access rights on a POSIX-like
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filesystem.
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setattr: called by the VFS to set attributes for a file. This function is
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called by chmod(2) and related system calls.
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setattr: called by the VFS to set attributes for a file. This method
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is called by chmod(2) and related system calls.
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getattr: called by the VFS to get attributes of a file. This function is
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called by stat(2) and related system calls.
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getattr: called by the VFS to get attributes of a file. This method
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is called by stat(2) and related system calls.
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setxattr: called by the VFS to set an extended attribute for a file.
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Extended attribute is a name:value pair associated with an inode. This
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function is called by setxattr(2) system call.
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Extended attribute is a name:value pair associated with an
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inode. This method is called by setxattr(2) system call.
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getxattr: called by the VFS to retrieve the value of an extended attribute
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name. This function is called by getxattr(2) function call.
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getxattr: called by the VFS to retrieve the value of an extended
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attribute name. This method is called by getxattr(2) function
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call.
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listxattr: called by the VFS to list all extended attributes for a given
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file. This function is called by listxattr(2) system call.
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listxattr: called by the VFS to list all extended attributes for a
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given file. This method is called by listxattr(2) system call.
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removexattr: called by the VFS to remove an extended attribute from a file.
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This function is called by removexattr(2) system call.
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removexattr: called by the VFS to remove an extended attribute from
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a file. This method is called by removexattr(2) system call.
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The Address Space Object
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========================
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The address space object is used to identify pages in the page cache.
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struct address_space_operations
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===============================
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-------------------------------
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This describes how the VFS can manipulate mapping of a file to page cache in
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your filesystem. As of kernel 2.6.13, the following members are defined:
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@ -502,8 +512,14 @@ struct address_space_operations {
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it. An example implementation can be found in fs/ext2/xip.c.
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The File Object
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===============
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A file object represents a file opened by a process.
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struct file_operations
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======================
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----------------------
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This describes how the VFS can manipulate an open file. As of kernel
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2.6.13, the following members are defined:
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@ -661,7 +677,7 @@ of child dentries. Child dentries are basically like files in a
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directory.
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Directory Entry Cache APIs
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Directory Entry Cache API
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--------------------------
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There are a number of functions defined which permit a filesystem to
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@ -880,3 +896,22 @@ Papers and other documentation on dcache locking
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1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
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2. http://lse.sourceforge.net/locking/dcache/dcache.html
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Resources
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=========
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(Note some of these resources are not up-to-date with the latest kernel
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version.)
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Creating Linux virtual filesystems. 2002
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<http://lwn.net/Articles/13325/>
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The Linux Virtual File-system Layer by Neil Brown. 1999
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<http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
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A tour of the Linux VFS by Michael K. Johnson. 1996
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<http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
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A small trail through the Linux kernel by Andries Brouwer. 2001
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<http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
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